Electrostatic printing with density control provided by charge measuring means



Jan. 31, 1967 w. E. JOHNSON 301,179

ELECTROSTATIC PRINTING WITH DENSITY CONTROL PROVIDED BY CHARGE MEASURINGMEANS Filed March 15, 1965 2 Sheets-Sheet 1 RI/ Fe. 2

'5 II II HZ j QTTORNEKS 3,301,179 IDED Jan. 31, 1967 w. E. JOHNSONELECTROSTATIC PRINTING WITH DENSITY CONTROL PROV BY CHARGE MEASURINGMEANS 2 Sheets-Sheet 2 Filed March 15, 1965 FORMERG REcT/F/EIZ FfluuTzaDjc/LLHTO HMPLlF/E Z Firm/ lak Scum/Ale 1| IIII United atent Ofiice3,301,179 ELECTROSTATIC PRINTING WITH DENSITY CONTRGL PROVIDED BY CHARGEMEAS- URING MEANS William E. Johnson, Temperance, Mich, assignor tOwens-Illinois Inc, a corporation of Ohio Filed Mar. 15, 1965, Ser. No.439,799 Claims. (Cl. 101114) This invention relates to electricalprinting methods and apparatus, and more particularly to improvements inmethods and apparatus for electrically transferring printing powderparticles from a supply source or bed through an image-defining aperturein a stencil screen to an article surface by the application of anelectric field.

A typical decorating process to which the present invention hasapplication is that disclosed in my copending application Serial No.393,817, filed August 31, 1964, and assigned to the assignee of thepresent application.

In one exemplary process of that type, a bed of printing powderparticles is supported upon an electrically conductive plate in spacedrelationship beneath a stencil screen having an image defining apertureor opening in the screen. An article to be decorated is placed inregistry with the screen apertures in spaced relationship above thescreen. Electric potential source means are connected between theconductive plate, screen and article to establish an electric fieldoperable to electrically charge the particles in the supply and impelthe particles from the supply upwardly through the screen aperture tothe surface of the article.

Experience in the practice of the process outlined above has shown thatwhere several articles in succession are decorated from the same powderbed, the density or thickness of the image layer applied to the articledecreases as the number of articles increase even though an adequatesupply of powder particles remains in the bed. The bed is initiallyprepared by placing a loose, unpacked bed of particles in a layer ofuniform thickness upon a fiat conductive plate. After several articleshave been decorated from the same bed, it has been observed that theparticles in the bed tend to become packed to some extent and it isbelieved that this pack ing action makes it more difficult for particlesto be transferred from the bed.

Accordingly, it is one object of the present invention to providemethods and apparatus for achieving a uniform image density on each of aplurality of articles decorated in succession from a powder bed.

. It is another object of the present invention to provide methods andapparatus for automatically controlling the application of the electricfield in a decorating process of the type referred to above to assurethat a uniform quantity of printing powder particles is applied to thearticle in each decorating operation, regardless of variations in thedecorating conditions.

The foregoing objects and features of the invention will become apparentby reference to the following specification and to the drawings.

In the drawings:

FIGURE 1 is a schematic diagram of one form of the invention;

FIGURE 2 is a schematic diagram of another form of the invention;

FIGURE 3 is a block diagram of one form of electric control circuitembodying the invention; and

FIGURE 4 is a wave diagram representing signals generated at variouspoints in the circuit of FIGURE 3.

In FIGURE 1 there is schematically disclosed an arrangement operable toapply an image-shaped layer of printing powder particles to the lowersurface of an article A supported as by supports schematicallyillustrated at 10. A bed 12 of printing powder particles is supportedupon an electrically conductive support member 14 and a stencil screen16 having image defining apertures such as 18 is interposed between thepowder bed 12 and the article A.

Screen 16 is preferably constructed of a relatively fine wire mesh whichis coated with a coating which fills the openings in the screen. Imageapertures 18 are formed in the screen by removing the coating fromselected portions of the screen, the openings being crisscrossed by thewire mesh. For details of suitable materials and methods for preparingsuch screens, reference may be had to United States Patent No.3,100,150.

These printing powder particles in bed 12 may take the form of finelypowdered particles of a glass frit, many suitable frits beingcommercially available.

A voltage source V1, grounded at one output terminal, is connected by anoutput lead 26 to powder bed support 14, while a second voltage sourceV2 is electrically connected through one output lead 22 to the articleA. The other output lead 24 of voltage source V2 is connected toelectrical ground through a black box 26 whose function will bedescribed in greater detail below. Screen 16 is also connected toelectrical ground.

Voltage sources V1 and V2 are operable, when energized, to establish anelectric field which extends from powder bed support 14 to the surfaceof article A, by electrically charging support member 14, screen 16 andarticle A to different electric potentials. The electric field strengthis chosen to be high enough to electrically charge the particles in bed12 and to electrically impel them upwardly through the apertures ofscreen 16 to the surface of article A. Further details of electric fieldstrength, particle size, screen mesh and the spacing between the bed,screen and article surface are set forth in my co-pending applicationSerial No. 393,817 referred to above.

In the normal operation of electrically transferring particles from thebed 12 to the surface of article A, many particles rebound from thescreen back to the bed. This action results in a packing of the powderbed which becomes more and more pronounced as successive articles aredecorated from the same powder bed. Because of this packing of thepowder bed, it becomes more and more difficult to transfer particlesfrom the bed to the article surface, as the number of articles insuccession decorated from the same bed increases.

This effect becomes apparent when an apparatus such as that disclosed inFIGURE 1 is operated to decorate a number of articles in succession byenergizing the electric field for a given amount of time for eacharticle. The first articles decorated under these circumstances aredecorated with images of satisfactory density, but as the number ofsuccessive articles increases, the density of the image applieddecreases. In order to achieve a uniform density in images applied toseveral successive articles from the same powder bed, it is necessary toincrease the time period of application of the electric field as thenumber of articles decorated increases. Unfortunately, the packingeffect on the powder bed, while having a general trend to increase thedifiiculty in transferring particles with the increase in number oftransfers, does not increase in a predictable manner.

In order to achieve uniform image density over a series of particlesdecorated successively from the same powder bed, it is necessary toregulate the time period of energization of the electric field inaccordance with the number of particles which have been accumulated onthe article surface.

The charging of a particle in powder bed 12 and the subsequent transferof the charged particle from the bed to the surface of article A can beconsidered 'as a flow of electric current in that a unit of electriccharge is physically transferred from support 14 to article A.

The function of black box 26 which is connected in series between screen16 and article A is to measure the flow of current through this circuit.Because current flow in this circuit, after the transient chargingcurrent, can be induced only by the transfer of a particle from screen18 to the surface of article A, current flow through this circuit is ameasurement of the number of unit electric charges transferred fromscreen 16 to the surface of article A and hence a measurement of thenumber of charged particles which reach article A. By integrating thecurrent, a measurement proportional to the total number of particlesaccumulated at any given time upon article A is obtained. By employingthe integrated current -to generate a signal when the number ofparticles on the surface of article A reaches a predetermined number,corresponding to the desired image density, the signal thus generatedmay be applied, through an appropriate switching device 28, toautomatically shut off the voltage sources to discontinue the electricfield.

In this manner, the electric field is automatically maintained until apredetermined number of particles have been transferred to the articlesurface, regardless of the amount of time required to effect thetransfer.

In FIGURE 3, an electrical block diagram of 'a suitable electricalarrangement for performing the functions of the FIGURE 1 embodiment isdisclosed, the various units being enclosed in broken line enclosuresdesignated V1, V2, 26 and 28 to indicate the approximate correspondenceof the various elements to those more generally described in thedescription of FIGURE 1 above. In FIGURE 3, the article A, stencilscreen 16, powder bed 12 and its support 14 appear in the upper righthand portion of the figure.

A decorating operation is initiated in the circuit of FIGURE 3 bymanually switching a pulse switch 30 in the upper left hand corner ofthe diagram from a normal or inactive contact 32 to a pulse contact 34.When switch 30 is shifted from contact 32 to contact 34, one level of avoltage source 36 is grounded at the juncture of resistors R4 and R5,and this voltage change is applied by capacitor C1 across resistor R1 as'a negative peak indicated at T=O on curve I of FIGURE 4. This negativepulse is applied to a monostable multivibrator of the one shot typeindicated at 38 which shifts the multivi-brator from one of its twopossible conductive conditions to the other. Multivibrator 38 may takenthe form of a standard circuit module sold by Engineering ElectronicsCompany of Santa Ana, California, under Catalog No. Z8889. This actionof the multivibrator activates squaring circuit 40 (EngineeringElectronics Company Catalog No. 290001) whose output is fed through twocathode follower buffer amplifiers 42 (Engineering Electronics CompanyCatalog No. Z8309) and is applied as a constant potential input to agate circuit 44, one of the cathode follower amplifier output wavesbeing indicated at curve II of FIGURE 4. Gate 44 may take the form of amore or less conventional diode gate circuit such as those explained inPulse and Digital Circuits, by Millman and Taub, 1956 Ed., section 147.

When these outputs are applied to gate 44, gate 44 is operable toconduct the output of a conventional 2 tube RC feedback oscillator 46 toa power amplifier 48 which amplifies the A.C. wave form oscillatoroutput indicated at curve III in FIGURE 4. Amplifier 48 may be anysuitable industrial type amplifier such as the Mark III 60 watt Dynakitamplifier sold by Dynaco, Inc. of Philadelphia, Pennsylvania. Theamplified A.C. wave is then passed through 'a high voltage transformerand rectifier 50 whose constant voltage output, curve IV of FIGURE 4, isapplied directly to powder support 14.

Low voltage source V2 is normally maintained ener- 4 gized at all timesand hence, upon the. application of the high voltage output oftransformer and rectifier 50 to powder support base 14, the particles inbed 12 become electrically charged and attracted to the article surfaceA as described in more detail above.

As the charged particles arrive at the surface of article A, they loseor give up their charge to the article and the charge thus imparted tothe article is accumulated upon an integrating capacitor C3, one side ofwhich became isolated from ground by the original shifting of pulseswitch 30 which initiated the powder transfer. As the charge isaccumulated on capacitor C3, the voltage begins to accumulate asindicated by curve V of FIGURE 4. The initial step on this curve is dueto the low voltage power supply and the fact that there is a finiteelectrical resistance between article A and stencil 16.

During the powder transfer, capacitor C3 is connected to an amplifiercircuit 52 which is normally held non-conductive by a bias voltage asfrom a bias voltage source 54. When the voltage across capacitor C3reaches a pre-selected value determined by the bias voltage applied toamplifier 52, the bias is overcome and the amplifier becomes conductive.A small portion of the output of oscillator 46 is fed into amplifier 52,and when the amplifier becomes conductive, upon the reaching of apredetermined voltage drop across capacitor C3, this portion of theoscillator output is passed through the amplifier and fed into asquaring circuit 56 (Engineering Electronic Company Catalog No. Z9000l).The amplifier output curve is indicated in FIGURE 4 at VI, and theresultant output of squaring circuit 56 is indicated at curve VII. Theoutput of the squaring circuit is applied to multivibr'ator 38 to shiftit back to its original conductive condition, thereby breaking thecircuit to the elements designated generally as V1 in FIGURE 3. As willbe noted from FIGURE 4, this action terminates the application of highvoltage to powder base 14 as indicated in curve IV.

The waves indicated in curves V, VI, and VII of FIG- URE 4 do not returnto zero at this time, because a voltage drop still exists acrosscapacitor C3 and is maintained until pulse switch 30 is manuallyreturned to its original position in contact with contact 32. Thisaction grounds both sides of capacitor C3, thereby dissipating thevoltage .drop across the capacitor.

Normally, under conditions such as those described in my co-pendingapplication Serial No. 393,817, images of desired density can be formedwith the high voltage power supply energized for time periods in theneighborhood of milliseconds. As stated above, because of the effect ofsuccessive transfers upon the condition of the powder bed, the timenecessary to obtain an image of desired density upon the articleincreases with successive transfers. By the arrangement disclosed above,the time during which the high voltage is applied to powder base 14 isautomatically determined by the accumulation of an electric charge uponthe capacitor C3 of the FIGURE 3 circuit, this accumulation of chargebeing directly proportional to the number of particles which have beenaccumulated on the article surface.

In the event a satisfactory image has not been accumulated on thearticle within a given amount of time, the cycle may be automaticallyterminated by connecting a second capacitor C2 to multivibrator 38 toperform the reversing function after a predetermined interval determinedby the time constant of the capacitor.

A second arrangement is disclosed in FIGURE 2 in which the potentialapplied to the article, screen and powder bed support 14 is suppliedfrom a single voltage source V3 as shown. In the FIGURE 2 arrangement,voltage source V3 has its opposite output terminals connected to powderbed support 14 as by conductor 60' and to a common point 62, which isgrounded, by a second conductor 64. Screen 16 is connected to commonpoints 62 by a circuit which includes a variable resistor R10 and aZener diode Z connected in parallel with each other.

Common point 62 is connected to the article A through a second circuit66 which includes a resistance R11 and a capacitor C5 connected inparallel with each other. A conductor 68 also connects article A to anon-ofi control device 70, which may be similar in general to thecorresponding portions of the circuit of FIGURE 3. Element 70 isemployed to automatically energize and de-energize voltage source V3.

The parallel circuit containing resistor R and Zener diode Z functionsto maintain screen 16 at a precisely regulated potential determined bythe characteristics of the Zener diode and the adjusted resistance ofvariable resistor R19.

While various embodiments of the invention have been disclosed anddescribed in detail above, it will be apparent to those skilled in theart that these embodiments may be modified. Therefore, the foregoingdescription is to be considered exemplary rather than limiting, and thetrue scope of the invention is that defined in the following claims.

I claim:

1. The method of applying a layer of powder particles to an articlesurface in a predetermined pattern comprising the steps of interposing astencil screen having a pattern defining aperture therethrough between asupply of powder particles and the surface of an article, connectingelectric potential source means to said supply, screen and articleoperable when energized to electrically charge particles in said supplyand impel charged particles from the supply through the aperture of saidscreen to the article surface, energizing said electric potential sourcemeans, and de-energizing the potential source means upon theaccumulation of a predetermined electrical charge on the article surfacedue to the accumulation of charged particles upon the article surfacerepresentative of a desired thickness of the layer of particles on thearticle.

2. The method of applying a layer of powder particles to an articlesurface in a predetermined pattern comprising the steps of interposing astencil screen having a pattern defining aperture therethrough between asupply of powder particles and the surface of an article, connectingelectric potential source means to said supply, screen and articleoperable when energized to establish an electric field capable ofelectrically charging particles in said sup ply and impelling thecharged particles from the supply through the aperture of said screen tothe article surface, energizing said electric potential source means,continuously measuring the accumulation of electrical charge upon thearticle surface while said potential means is energized, andde-energizing the potential source means in response to the accumulationof a pre-determined electrical charge upon the article surfacerepresentative of a preselected thickness of the layer of particles onthe article surface.

3. The method of applying powder particles in an image-shaped layer ofpredetermined thickness to an article surface comprising the steps ofinterposing a stencil screen having an image defining aperturetherethrough between a supply of printing powder particles and thesurface of an article, connecting electric potential source means tosaid supply screen and article operable when en ergized to establish afirst electric potential difference between said supply and said screenand to establish a second electric potential dilference between saidscreen and said article, said potential differences being of a magnitudesufiicient to electrically charge particles in said supply and impelcharged particles from the supply through the aperture of said screen tothe article surface, energizing said electric potential source means,measuring the variation in said second potential difference due to thetransfer of charged particles from said screen to the article surface,and de energizing the potential source means when said second potentialdifference reaches a value representative of the accumulation of apredetermined thickness of the image-shaped layer of particlesaccumulated upon the article surface.

4. The method of applying a layer of powder particles to an articlesurface in a predetermined pattern comprising the steps of interposing astencil screen having a pattern defining aperture therethrough between asupply of powder particles, and the surface of an article applyingelectric potential pulses to said supply, screen and article toelectrically charge particles in said supply and impel charged particlesfrom the supply through the aperture of said screen to the articlesurface, and regulating the time duration of the application of saidelectric pulses to terminate upon the accumulation of a predeterminedelectrical charge on said article surface due to the transfer of chargedparticles from said supply to said article surface.

5. The method of applying an image-shaped layer of printing powderparticles to an article surface comprising the steps of interposing astencil screen having an image defining aperture therethrough between asupply of printing powder particles and the surface of an article,connecting electrical potential source means to said supply screen andarticle operable when energized to establish an electric field capableof electrically charging particles in said supply and impelling thecharged particles from the supply through the aperture of said screen tothe article surface, energizing said electric potential source means,measuring the electric charge transferred to said article surface. byparticles accumulating upon the article surface while said potentialmeans is energized, and de-energizing the potential source means whenthe total electric charge transferred to said article surface by theparticles accumulated thereon reaches a preselected minimum magnitude.

6. In an apparatus for applying an image-shaped layer of printing powderparticles to an article surface including a stencil screen having animage defining aperture therethrough, means for supporting a supply ofprinting powder particles at one side of said screen, means forsupporting an article to which the particles are to be supplied at theopposite side of said screen, electric potential source means operablewhen energized to establish an electric field between said supply andarticle capable of electrically charging powder particles in said supplyand impelling the charged particles from said supply through theaperture of said screen to the surface of the article, and means forenergizing said electric potential source means; the improvementcomprising means for measuring the variation of electric potentialbetween said screen and said article due to the accumulation of chargedparticles on said article while said potential source means .isenergized, and means operable by said measuring means for de-energizingsaid potential source means when the potential difference between saidscreen and article reaches a preselected magnitude.

7. In an apparatus for applying an image-shaped layer of printing powderparticles to an article surface including a stencil screen having animage defining aperture therethrough, means for supporting a supply ofprinting powder particles at one side of said screen, means forsupporting an article to which the particles are to be applied at theopposite side of said screen, electric potential source means operablewhen energized to establish an electric field between said supply andarticle capable of electrically charging powder particles in said supplyand impelling the charged particles from said supply through theaperture of said screen to the surface of the article, and means forenergizing said electric potential source means; the improvement whereinsaid potential source means comprises a first voltage source means forapplying a first potential difference between said supply and saidscreen, second voltage source means for applying a second potentialdifference between said screen and said article, and means for measuringvariations in said second potential difference due to the passage ofcharged particles from said screen to said article.

8. In an apparatus for applying an image-shaped layer of printing powderparticles to an article surface including a stencil screen having animage defining aperture therethrough, means for supporting a supply ofprinting powder particles at one side of said screen, means forsupporting an article to which the particles are to be applied at theopposite side of said screen, electric potential source means operablewhen energized to establish an electric field between said supply andarticle capable of electrically charging powder particles in said supplyand impelling the charged particles from said supply through theaperture of said screen to the surface of the article, and means forenergizing said electric potential source means; the improvement whereinsaid potential source means comprises a voltage source having two outputterminals, means connecting one output terminal to said supply, a firstelectric circuit connecting the other output terminal of said source tosaid article, and a second electric circuit connecting said other outputterminal to said screen, said second circuit including an electricalresistance and a Zener diode connected in parallel with each otherbetween said other output terminal and said screen.

9. In an apparatus as defined in claim 8; the further improvementwherein said first circuit comprises a resistor and a capacitorconnected in parallel with each other between said other output terminaland said article, and switching means responsive to the accumulation ofa predetermined charge on said capacitor for de-energizing said electricpotential source means.

10. In an apparatus for applying an image-shaped layer of printingpowder particles to an article. surface including a stencil screenhaving an image defining aperture therethrough, means for supporting asupply of printing powder 8 particles at one side of said screen, meansfor supporting an article to which the particles are to be applied atthe opposite side of said screen, electric potential source meansoperable when energized to establish an electric field between saidsupply and article capable of electrically charging powder particles insaid supply and impelling the charged particles from said supply throughthe aperture of said screen to the surface of the article and means forenergizing said electric potential source means; the improvement whereinsaid electric potential source means comprises first means for applyinga potential pulse of predetermined magnitude to said supply, secondmeans for applying a potential pulse of a predetermined magnitude tosaid article, and means for de-energizing said potential source meanswhen a sufiicient number of electrically charged particles haveaccumulated upon said article to change the electric potential on saidsurface to a predetermined potential.

References Cited by the Examiner UNITED STATES PATENTS 2,987,037 6/1961Bolton 118637 2,996,575 8/1961 Sims. 3,081,698 3/1963 Childress et al.10l129 3,218,967 11/1965 Childress 1011 14 3,218,968 11/1965 Childresset al. 101-115 FOREIGN PATENTS 81,920 9/ 1956 Denmark.

ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner.

6. IN AN APPARATUS FOR APPLYING AN IMAGE-SHAPED LAYER OF PRINTING POWDERPARTICLES TO AN ARTICLE SURFACE INCLUDING A STENCIL SCREEN HAVING ANIMAGE DEFINING APERTURE THERETHROUGH, MEANS FOR SUPPORTING A SUPPLY OFPRINTING POWDER PARTICLES AT ONE SIDE OF SAID SCREEN, MEANS FORSUPPORTING AN ARTICLE TO WHICH THE PARTICLES ARE TO BE SUPPLIED AT THEOPPOSITE SIDE OF SAID SCREEN, ELECTRIC POTENTIAL SOURCE MEANS OPERABLEWHEN ENERGIZED TO ESTABLISH AN ELECTRIC FIELD BETWEEN SAID SUPPLY ANDARTICLE CAPABLE OF ELECTRICALLY CHARGING POWDER PARTICLES IN SAID SUPPLYAND IMPELLING THE CHARGED PARTICLES FROM SAID SUPPLY THROUGH THEAPERTURE OF SAID SCREEN TO THE SURFACE OF THE ARTICLE, AND MEANS FORENERGIZING SAID ELECTRIC POTENTIAL SOURCE MEANS; THE IMPROVEMENTCOMPRISING MEANS FOR MEASURING THE VARIATION OF ELECTRIC POTENTIALBETWEEN SAID SCREEN AND SAID ARTICLE DUE TO THE ACCUMULATION OF CHARGEDPARTICLES ON SAID ARTICLE WHILE SAID POTENTIAL SOURCE MEANS ISENERGIZED, AND MEANS OPERABLE BY SAID MEASURING MEANS FOR DE-ENERGIZINGSAID POTENTIAL SOURCE MEANS WHEN THE POTENTIAL DIFFERENCE BETWEEN SAIDSCREEN AND ARTICLE REACHES A PRESELECTED MAGNITUDE.